A polarizing plate having light transmission and shielding functions is a basic constituent factor, together with a liquid crystal having a light switching function, of a liquid crystal display (LCD). The application use of LCD has been extended generally from that for small sized equipments such as pocket calculators and watches at the time of initial stage to that for notebook type personal computers, word processors, liquid crystal projectors, liquid crystal television sets, car navigations and measuring instruments for indoor and outdoor use, and they are used under wide working conditions including from low temperature to high temperature and low humidity to high humidity, so that a polarizing plate having high polarizing performance and excellent durability has been demanded.
At present, a polarizing plate is manufactured by incorporating iodine or dichroic dye as a polarizer in a stretched and oriented a film of polyvinyl alcohol or a derivative thereof. Alternatively, it is manufactured from a polyene type film by preparing a polyene through dehydrochlorination of a polyvinyl chloride film or dewatering of a polyvinyl alcohol type film and then orienting the same. Among them, the iodine type polarizing plate using iodine as a polarizer is excellent in the initial polarizing performance but is sensitive to water and heat and involves a problem in the durability when it is used under high temperature and high humidity state for a long time. For improving the durability, methods of treating with formalin or a boric acid-containing aqueous solution or using a polymer film of low moisture permeability as a protective film has been considered but this is not yet sufficient. On the other hand, a dye type polarizing plate using a dichroic dye as the polarizer is excellent in moisture proofness and heat resistance compared with the iodine type polarizing plate but the initial polarizing performance is not generally sufficient.
Further, in a polarizing plate of a neutral color prepared by adsorption and orientation of several kinds of dichroic dyes in a polymer film, when light leakage (color leakage) occurs at a specified wavelength in a visible light region, particularly, in a wavelength region from 400 to 700 nm in a state of stacking two polarizing plates such that their orienting directions are perpendicular to each other (crossed state), hue of the liquid crystal display sometimes changes in the dark state when the polarizing plate is attached to a liquid crystal panel. In view of the above, for preventing discoloration of the liquid crystal display due to color leakage for a specified wavelength in the dark state when the polarizing plate is attached to the liquid crystal display device, the transmittance in the crossed state (transmittance for crossed state) has to be lowered uniformly in a visible light region, particularly in a wavelength region from 400 to 700 nm.
Further, in a case of a color liquid crystal projection type display, that is, a color liquid crystal projector, a polarizing plate is used for the liquid crystal image forming portion. However, the brightness is reduced inevitably due to significant absorption of light by the polarizing plate and magnification of an image of a small area of 0.9 to 6 inch into about several tens inch to one hundred and several tens inch, so that a light source at a high luminosity is used. In addition, there is a continuous demand for the further increase of the brightness of the liquid crystal projector and, as a result, the intensity of the light source used increases more and more.
By the way, a neutral gray iodine type polarizing plate having satisfactory polarizing performance has generally been used as a polarizing plate for a liquid crystal image forming area of a color liquid crystal projector. However, the iodine type polarizing plate involves a problem that light fastness, heat resistance and wet heat resistance are not sufficient because the polarizer is iodine. In order to solve the problem, a neutral gray polarizing plate using a dye type dichroic colorant as a polarizer has been used. In the neutral gray polarizing plate, dyes of trichromatic colors are used in combination in order to improve the transmittance and the polarizing performance in average in the entire visible ray wavelength region (400 to 700 nm). Accordingly, to the market's demand for more brightness as in the color liquid crystal projector, the light transmittance is poor and it involves a problem that the intensity of the light source has to be increased more to attain bright display. In order to solve the problem, three polarizing plates, that is, for blue channel, green channel and red channel corresponding to the trichromatic colors have become used.
As the dye used for the manufacture of a polarizing plate, Japanese Patent Publication No. 2844360 describes, for example, a water soluble dye of the following formula (7) in Example 1.

However, the polarizing plate containing the existent water soluble dye described above has not yet completely satisfied the needs of users sufficient in view of the polarization characteristics, absorption wavelength region and the hue.
Further, among the three polarizing plates corresponding to trichromatic colors of a color liquid crystal projector, that is, for blue channel, green channel and red channel, the polarizing plate for use in the green channel (green light) lacks both in favorable brightness and polarizing performance and improvement therefor has been demanded.
An object of this invention is to provide a high performance polarizing plate having excellent polarizing performance, moisture proofness and heat resistance. Further, another object of the invention is to provide a high performance neutral color polarizing plate formed by adsorbing and orienting two or more kinds of dichroic dyes in a polymer film, which causes no color leakage at the crossed state in a visible light region, particularly, in a wavelength region of from 400 to 700 nm and has excellent polarizing performance and moisture proofness and heat resistance.
A further object is to provide a high performance polarizing plate excellent both in brightness and polarizing performance as a polarizing plate for use in green channel (green color light) among the three polarizing plates corresponding to the trichromatic colors of a colored liquid crystal projector.